This invention relates to treatment of waste gases contaminated with nitrogen trifluoride, NF.sub.3. In particular, the invention relates to a process for treating a gaseous stream comprising NF.sub.3 and optionally other acidic gaseous fluoride substances and to a reducing agent comprising an Fe--Cr-containing alloy useful for destroying NF.sub.3 in such a process.
Nitrogen trifluoride, NF.sub.3, has been very useful as an oxidizer for high-energy fuels used in rocketry or the like and as a fluorine source for a certain type of chemical laser. Recently, NF.sub.3 has been conveniently employed as a dry-etching gas as well as a cleaning gas in the very large scale integration (VLSI) industry, because it does not leave any harmful residues after etching and cleaning treatments.
Normally, NF.sub.3 is present as a nonflammable gas that is very stable and nonreactive with water, acids and aqueous alkalis at normal temperatures. However, it is a toxic substance having a threshold limit value, TLV, of 10 ppm. Therefore, it is necessary to treat hazardous NF.sub.3 -containing gases to eliminate or substantially reduce the toxicity before releasing the used gases into the environment. For this purpose, the following two methods have been proposed.
i) Method for treating the contaminated gases with hot metals:
In this method, NF.sub.3 is allowed to react with a metallic material at raised temperatures to produce a metal fluoride. If the fluoride is a solid substance forming a coating on the surface of the metallic material, the reaction is inhibited by growth of the coating so that the capacity of decomposing the NF.sub.3 contaminant becomes seriously hindered after a short period. On the other hand, if a powdery or flaky fluoride is produced by the reaction between NF.sub.3 and the metal, then the powder tends to block the treatment vessel and pipings. If the product fluoride is gaseous, the gas should be subjected to an appropriate secondary treatment, such as physical absorption, scrubbing or the like, requiring a larger apparatus as a whole and incurring additional operation costs. The gaseous fluoride product may generally be corrosive to the treatment apparatus, giving rise to another serious problem.
ii) Method for treating the contaminated gases with active carbon at raised temperatures:
When this method is carried out above 400.degree. C., the contaminant NF.sub.3 can be decomposed relatively effectively, but this is accompanied with a large proportion of a by-product dinitrogen difluoride, N.sub.2 F.sub.2, which itself is highly toxic and also detonative. In order to inhibit formation of the by-product N.sub.2 F.sub.2, the period during which NF.sub.3 contacts with the active carbon should be extended and also the reaction temperature should be raised to 500.degree. C. or higher. However, even under these conditions it is impossible to completely prevent the formation of the by-product. Further in this case, fluorocarbons such as CF.sub.4, C.sub.2 F.sub.6 and the like could be formed also as by-products, which will provide a hazardous mixture with any residual NF.sub.3. The mixture tends to detonate sympathetically with N.sub.2 F.sub.2. If the gases to be treated contain O.sub.2, the active carbon can be burnt therewith under the process conditions.
The above prior art methods thus do not appear to be feasible in commercial practice.
In industrial applications, NF.sub.3 is used in various concentrations appropriate for respective applications. When diluted, an inert gas, typically N.sub.2 is used as a diluent.
Depending upon the applications, the waste gases may contain, in addition to any residual NF.sub.3, other gaseous substances, for example, acidic fluoride such as HF, SiF.sub.4, WF.sub.6, MoF.sub.6 or the like that are by-products in the respective applications. For example, HF and SiF.sub.4 are usually found in waste gas streams from the NF.sub.3 -dry-etching processes in the VLSI industry.
The prior art methods could not be applied successfully to sweeten the waste gases containing NF.sub.3 in combination with acidic gaseous fluoride substances, such as HF, SiF.sub.4, MoF.sub.6, WF.sub.6 or the like.
The present invention seeks to overcome the problems experienced in the prior art methods.